In the production of castings by gravity pouring, which may be called simply “pouring” below, a so-called sand mold, which is a gas-permeable casting mold formed by sand particles, is most commonly used. When a melt is charged into such a gas-permeable casting mold, which may be called simply “casting mold,” a gas (generally air) remaining in a cavity having a particular shape is discharged through the cavity surface, so that the cavity is fully filled with the metal melt, which may be called simply “melt” below, resulting in a casting having substantially the same shape as that of the cavity. The casting cavity generally comprises a sprue, a runner, a riser and a product-forming cavity in this order from the melt-supplying side. In conventional technology, pouring is completed by forming a melt head as high as filling a product-forming cavity in a sprue.
A solidified casting has a shape corresponding to combined shapes of a sprue, a runner, a riser and a product-forming cavity. The riser is not an unnecessary portion as a cavity for obtaining a good product, while the sprue and the runner are inherently unnecessary portions because they are merely paths for a melt to flow to the product-forming cavity. Accordingly, as long as a melt is solidified in a state of filling the sprue and the runner, drastic improvement in a pouring yield cannot be obtained. In a case where unnecessary cast portions are integrally connected to a cast product, unnecessary cast portions should be separated from the cast product in a subsequent step, resulting in low production efficiency. Accordingly, cast portions in the sprue and the runner pose a serious problem in gravity pouring.
JP 2007-75862 A and JP 2010-269345 A propose a method of drastically solving the above problem, which comprises gravity-pouring a melt in a volume smaller than that of the entire cavity and substantially equal to that of a desired cavity portion, part of a gas-permeable casting cavity which may be called simply “cavity,” to charge the metal melt into the desired cavity portion; supplying a compressed gas through a sprue before the poured melt is solidified, such that the desired cavity portion is filled with the melt; and then solidifying the melt. Because pressure provided by the melt head is obtained by the compressed gas by this method, it is expected that a melt need not exist in the sprue and the runner.